We are examining 4 specific hydroxylation reactions that occur in the metabolism of aromatic compounds in higher plants. These four reactions and the biological system which accomplishes these reactions are: (1) the ortho-hydroxylation of cinnamic acid by chloroplast preparations from M. alba. (2) the p-hydroxylation of cinnamic acid by microsomes from higher plants. (3) the N-hydroxylation of tyrosine, phenylalanine, valine and isoleucine by cyanophoric plants. (4) the formation of DOPA from tyrosine, an ortho-hydroxylation, that occurs in broad bean.